Satellites and other spacecraft are in widespread use for various purposes including scientific research and communications. These communications missions, however, cannot be accurately fulfilled without digital communications. In many applications, the satellite relies upon a form of digital communication referred to as Asynchronous Transfer Mode (ATM) to relay various information.
Asynchronous Transfer Mode (ATM) is one of the general class of digital switching technologies that relay and route traffic by means of a virtual circuit identifier (VCI) contained within the cell. Unlike common packet technologies, such as X.25 or frame relay, ATM uses very short, fixed length units of information, called cells. In applications utilizing ATM, packets at a source are first broken up into these fixed length packets (ATM cells), transmitted, and then reassembled at a destination. ATM cells are 53 bytes long. They consist of a 5-byte header (containing an identifier of data flow which implicitly identifies the source address and the destination address) and a 48-byte information field. The header of an ATM cell contains all the information the network needs to relay the cell from one node to the next over a pre-established route. User data is contained in the remaining 48 bytes.
ATM uses a concept of virtual networking (or channels) to pass traffic between two locations, establishing virtual connections between a pair of ATM end-systems which are needed to connect a source with a destination. These connections are termed “virtual” to distinguish them from dedicated circuits. ATM cells always traverse the same path from source to destination. However, ATM does not have to reserve the path for one user exclusively. Any time a given user is not occupying a link, another user is free to use it.
ATM connections exist only as sets of routing tables held in each network node, switch, or other intermediate system, based on the virtual circuit identifier (VCI) and virtual path identifier (VPI) contained in the cell header. When a virtual path is established, each node (or switch) is provided with a set of lookup tables that identify an incoming cell by header address, route it through the node to the proper output port, and overwrite the incoming VCI/VPI with a new one that the next node along the route will recognize as an entry in its routing table.
The cell is thus passed from switch to switch over a prescribed route, but the route is “virtual” since the facility carrying the cell is dedicated to it only while the cell traverses it. Two cells that are ultimately headed for different destinations may be carried, one after the other, over the same physical wire for a common portion of their journey.
Prior art satellite implementations use fixed beams which are configured so that they do not interfere with each other. With this setup, ATM switching functionality can be implemented in a manner which is functionally equivalent to traditional ground based ATM implementations. However, such methods do not have the ability to handle hopped spot beams where all beams use the same frequency spectrum (frequency reuse) in a satellite that has fewer spot beams than destinations. Generally, the frequency spectrum is reused multiple times between different spot beams in order to increase the utilization of the allocated frequency spectrum. For a satellite that has fewer spot beams than destination regions (or cells), the spot beams are in turn reused (i.e., hopped) to service multiple destination regions in a Time Division Multiplexed basis. Time division multiplexing uses fixed time intervals (slots) during which each spot beam transmits a burst of packets to its respective destination. Since spot beam destinations need to be chosen dynamically for each slot based on the packets scheduled out at that time, the switch needs to be able to carefully select the packets for each beam in each slot so that none of the spot beams spatially interfere with each other in the respective slot. Also, the switch needs to provide fairness with flexibility to prioritize bandwidth offered to customers appropriate to the specific business model for the system. The said flexibility needs to be achieved on a system that uses hopped spot beams and frequency reuse. Traditional ATM methods do not accommodate switching over links which compete with each other for physical properties and resources such as frequency isolation, transponder power, and bandwidth.
Another disadvantage of traditional ATM methods is that they are not capable of handling spot beam power constraints. The power required for transmitting packets can vary between destination regions due to a number of factors such as weather and altitude. Each spot beam is limited in its power output capability such that certain beams are more capable than others. Also, there would be a limit on the total power summed across all active spot beams. Therefore, the switch has to be able to schedule packets to spot beams in a manner that none of the spot beams exceed their individual power capacities as well as not to exceed the total power output capacity of the satellite for all spot beams combined for each Time Division slot.
The disadvantage associated with these conventional asynchronous transfer mode packet scheduling techniques have made it apparent that a new technique for spot beam hopping packet scheduling is needed. The new technique should be able to efficiently handle spot beam hopping constraints which traditional ATM implementations do not handle.